Factor VIII is the antihaemophilic factor in blood. Its actual structure is, at present, unknown though it is suspected to be a complex high molecular weight protein or possibly associated with such a protein. There is much interest in its extraction, e.g. from blood plasma, so that it can be used in the treatment of patients suffering from haemophilia. Factor VIII is, however, unstable and this has created difficulties in its extraction. Methods of extraction are, however, available such as a cryoprecipitation method. This method has the disadvantage of poor recovery (e.g. about 40%) of Factor VIII from the initial plasma and of giving rise to a dilute product, which accordingly creates problems of administration to the patient because of the large volume of product required to achieve suitable dose levels. The dilute product may, however, if required, be further purified and freeze dried to give a concentrated form of Factor VIII, but with further loss of Factor VIII activity.